1. Field of the Invention
This invention relates generally to a video tape recorder and, more particularly, to tracking control during an editing operation that can involve utilizing at least two such video tape recorders.
2. Description of the Prior Art
Editing recorded information typically involves the arrangement of one or more portions that have been previously recorded onto a single composite record medium, which is the so-called master tape. A master tape can be produced using a single source tape or a number of source tapes and such editing operations are referred to as assembly editing, in which the signal segment that is to be reproduced from one tape and subsequently recorded on the master tape is typically referred to as a cut. Thus, using assembly editing techniques, a number of cuts can be recorded sequentially on the so-called master tape. Therefore, assuming video information is involved, at least one video tape playback apparatus and at least one video tape recording apparatus are required to perform the assembly editing technique described above and, typically, these units are the well-known video tape recorders. Although video tape recorders are produced to high manufacturing standards, there are some measurable differences, which fall within the manufacturing tolerances, among all of the video tape recorders that are produced. For example, the phase of a video signal recorded by the rotary recording head will almost always be found to be slightly different than the phase of a control signal (CTL) that is recorded by a fixed control head. This phase difference is referred to hereinafter as CTL phase information. Another example of manufacturing discrepancies that may occur among video tape recorders is that the location of the head on the head mounting element, typically known as a dynamic tracking device (DT), can vary by a measurable amount even though the head locations are within manufacturing tolerances. Hereinafter, such differences in head mounting location that occur during manufacturing are referred to as head position information. Therefore, if assembly editing is achieved by moving the master tape from one video tape recorder to another video tape recorder, the differences of the CTL phase information or the head position information will cause an abrupt change in pitch between the last track of a cut and the first track of the next cut on the master tape, which will then adversely affect the tracking servo of a video tape recorder in the playback mode. The tracks in each individual cut will, of course, all have the same pitch and the tracking servo of the video tape recorder can provide correct tracking for each such cut.
In order to resolve the above-described problem it has been proposed to control the tracking by changing either the CTL phase or the head position, in accordance with tracking information obtained by reproducing a cut located prior to the desired editing point. This operation is aptly termed tracking adjustment, and it is generally possible to provide a smooth editing-in of the cut at the desired position. Nevertheless, disadvantages still remain in this proposed approach. More particularly, although an abrupt change of the head tracking can be prevented between the end of one cut and the beginning of the next cut at the editing point using the above-described tracking adjustment, there nevertheless is detected a small appreciable tracking discrepancy at the editing point between the last track and first track of successive cuts. This tracking discrepancy has been found to occur whether or not tracking adjustments are made using CTL phase information or dynamic tracking head information and is thought to be due to inherent errors in the head position detector used in tracking control and to the fact that there are no control signal pulses on the tape at the location at which the next cut is to be recorded. Although this tracking discrepancy initially appears to be a minor problem and, by itself, is not a serious problem, in an assembly editing operation a large number of cuts will typically be recorded on the master tape by the same video tape recorder, maybe up to two hundred cuts. Even using the same tape recorder a tracking adjustment as described above will be necessary and since the same video tape recorder is used the head position detector error at each editing point, for example, can be considered to be substantially the same. Therefore, the tracking discrepancy that occurs at each editing operation will accumulate, and the accumulated tracking discrepancy will become appreciable so as to be readily noticeable and objectionable. Not only will such accumulated discrepancies become noticeable but in the case of a video tape recorder with the head mounted on a dynamic tracking element, the result will be that the dynamic tracking element will be driven to the limit of its deflection, with no travel remaining to accommodate additional tracking discrepancies.